The present invention relates to an image processing apparatus for changing an image and, more particularly, changing transformation coefficients upon frequency transformation, its method, a program, and a storage medium.
With the recent advance of digital technologies, a radiation image is converted into a digital image signal, and that digital image signal undergoes an image process, thus displaying the processed signal on, e.g., a CRT or printing it out. Upon photographing a radiation image, the X-ray dose is preferably as small as possible in consideration of the influences on a patient. But an image sensed with a small X-ray dose contains many quantization noise components, which disturb diagnosis.
For this reason, processes for removing such noise have been conventionally examined. For example, a noise removal process using a simple median filter, a method of removing noise by extracting high-frequency components using a smoothed image, and the like are done. In recent years, a multiplex frequency process for removing noise by segmenting an input image into a plurality of frequency bands, and executing independent processes for respective frequency bands has been examined.
In a filter process for removing noise by extracting high-frequency components using a smoothed image, since a single frequency band is used, noise removal cannot be effectively made if noise components are distributed to a broad frequency band. To avoid this, a plurality of filters having different sizes (i.e., different frequencies) may be simultaneously used. However, computation cost required for the process increases considerably. In order to maintain optimal frequency characteristics of a filter for noise removal, filter size adjustment corresponding to an object is indispensable, resulting in poor versatility.
The above problems can be greatly reduced using the multiplex frequency process represented by, e.g., discrete wavelet transformation in noise removal. However, since the multiplex frequency process executes uniform frequency processes in all spatial directions, for example, it cannot distinguish an edge having a direction component from isolated point noise having no direction component, and it is difficult for that process to remove noise while preserving the edge.
The present invention has been made in consideration of the aforementioned problems, and has as its object to effectively execute noise removal.
In order to achieve the above object, for example, an image processing apparatus of the present invention comprises the following arrangement.
That is, an image processing apparatus comprises:
frequency transformation means for obtaining a plurality of subbands by computing frequency transforms of an image;
determination means for determining transformation coefficients in a subband to be changed using at least two subbands of the plurality of subbands obtained by the frequency transformation means; and
coefficient conversion means for converting the transformation coefficients determined by the determination means,
wherein the frequency transformation means reconstructs an image using transformation coefficients of all the subbands including the transformation coefficients converted by the coefficient conversion means.
The determination means further comprises:
threshold value setting means for setting threshold values for at least two subbands of the plurality of subbands obtained by the frequency transformation means; and
threshold value processing means for executing a threshold value process of transformation coefficients in the subbands for which the threshold values are set by the threshold value setting means, using the threshold values, and
the determination means determines the transformation coefficients in the subband to be changed using a threshold value processing result of the threshold value processing means.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.